Breaker point mechanism and cam therefor



May 30, 1967 s. M. TERRY BREAKER POINT MECHANISM AND CAM THEREFOR FiledJuly 16, 1965 I N VENTOR. B Y Jim/q fer/y MQW, 14M;

ATTORNEYS I United States Patent ration of Massachusetts Filed July 16,1965, Ser. No. 472,575 7 Claims. (Cl. 200-24) This invention relatesgenerally to breaker point mechanisms for magnetos and the like, anddeals more particularly with an improved cam for such a mechanism.

In the description which follows, the breaker point mechanism and thecam of this invention are shown as applied to a magneto similar to thatshown in US. Patent No. 2,847,492. It should be understood, however,that this is done 'by way of example only and that, while the inventionis particularly well suited for use in such a magneto mechanism, thereis no intention to limit the invention to this use and that it may beused to advantage in other electrical devices employing a similar camactuated mechanism for making and breaking an electrical circuit.

The general object of the invention is to provide an improved cam forthe breaker point mechanism of a magneto or the like which cam, incomparison to present cams of the same general type, is more readily andeasily mounted and aligned on a driving shaft and has improvedresistance to shaft vibration so as to be less subject to fretting andother wear and to resulting angular misalignment between the cam and theshaft.

A further object of the invention is to provide an improved cam of thetype mentioned in the foregoing paragraph which may be manufactured atrelatively low cost by the use of powdered metal techniques.

Other objects and advantages of the invention will be apparent from thefollowing description and from the drawings forming a part hereof.

The drawing shows a preferred embodiment of the invention and suchembodiment will be described, but it will understood that variouschanges may be made from the construction disclosed, and that thedrawing and description are not to be construed as defining or limitingthe scope of the invention, the claims forming a part of thisspecification being relied upon for that purpose.

Of the drawing:

FIG. 1 is a transverse sectional View of a magneto including a breakerpoint mechanism and cam embodying the invention, the view being takenalong the line 11 of FIG. 2 with the cover plate of the breaker pointmechanism housing being omitted.

FIG. 2is a longitudinal sectional view taken along the line 22 of FIG.1.

FIG. 3 is a perspective view of the cam employed in the FIG. 1 magnetomechanism.

FIG. 4 is an elevational view of the cam employed in the FIG. 1 magnetomechanism.

FIG. 5 is a fragmentaryelevational view showing a cam and an associatedbreaker point mechanism of the type employed in FIG. 1.

Turning now to the drawing for a detailed description of the invention,reference numeral indicates, in general, a magneto having a breakerpoint mechanism embodying this invention. The magneto 10 may takevarious different forms and in the presently illustrated case is shownby Way of example as being generally similar to the one shown in PatentNo. 2,847,492 to which reference may be made for further details of itsconstruction. For the present, it is sufficient to note that the magnetoincludes a stator adapted for attachmentto an engine frame or plate 12having a hub 14 which rotatably receives a shaft 16, the shaftprojecting through and beyond thehub as shown in 3,322,909 Patented May30, 1967 is preferably made of a stack of laminated sheet material so astoreduce eddy current and hysteresis losses. Screws 21, 21 passingthrough the core and threaded into bosses, not shown, on the engineframe hold the core in place. The core 18 provides three spaced andgenerally parallel poles 20, 20 having arcuate end faces 22, 22 whichare concentric with the axis of the shaft 14 and equi-distant therefrom.A coil assembly 24 is received on the center stator pole 20 and includesprimary and secondary windings which surround the center pole and whichare made up of different number of turns of conducting wire inaccordance with conventional magneto practice. The primary coil issuitably grounded at one end and its other end is connected by means ofan insulated conductor 26 to a suitable breaker point mechanism. Thesecondary coil is also suitably grounded at one end and at its other endis connected with an insulated conductor 28 which in turn is connectiblewith the spark plug of the associated engine.

The magneto also includes a flywheel rotor 30 which is connected withthe shaft 16 so as to be rotatable therewith. The rotor is preferablyformed of a non-magnetic material and it is shown as having a disc-likewall having a central hub portion 31 and an annular peripheral flange 32which surrounds the stator. The inner or internal face of the flange 32is generally cylindrical in shape and embedded in the flange is apermanent magnet 34 and two associated pole pieces 36, 36. Duringrotation of the rotor, the pole pieces 36, 36 move across the statorpole faces 22, 22 and complete flux circuits through the stator, theflux passing through the center pole 20 suddenly reversing in directionat one point of the rotor rotation to induce voltages in the primary andsecondary coils in a manner well known in the art. The breaker pointmechanism is timed to break the circuit to the primary coil atapproximately the instant of maximum voltage induced therein, therebygenerating a high voltage in the secondary coil which is connected withthe spark plug.

The breaker point mechanism is mounted on the core 18 and includes asupporting box or housing 38 which is connected with the core 18 byrivets 40, 40, the box including a rear wall 42 and an axially extendingside wall 44. As shown in FIG. 2, the box is normally closed by a coverplate 46 which is shown removed in FIG. 1 to reveal other parts of thebreaker mechanism.

The breaker mechanism includes a fixed breaker point 48 carried by abracket 50 secured by a screw 52 to the rear wall 42 of the box. Amovable breaker point 54 is carried by a rocker arm 56 mounted on apivot pin 58 carried by the rear box wall 42. The rocker arm 56 includesa cam follower 60 which is engageable with a cam 62 carried by the shaft16 for moving the movable breaker point 54-between opened and closedpositions relative to the fixed breaker point 48. A spring 64 serves tobias the rocker arm to move the movable breaker point 54 into engagementwith the fixed breaker point 50. The spring, or an auxiliary conductorassociated therewith, also serves to provide an electrical connectionbetween the conductor 28 and the movable breaker point, and by means ofa conductor 66 a condenser 68 is connected and parallel with the breakermechanism in accordance with'standard practice.

The breaker point cam 62 is received on a cylindrical portion 70 of theshaft 16 which is of a somewhat smaller diameter than the inner portionof the shaft and is separated from such inner portion by a radialshoulder 72. Ad-

jacent the outer end of the cylindrical shaft portion 70 is a taperedportion 74. The tapered portion 74 has a major diameter equal to thediameter of the cylindrical portion 70 and receives the correspondinglytapered hub portion 31 of the rotor. Included in the shaft is an axiallyextending key way 76 which receives a key 78 included in the cam forangularly fixing the cam relative to the shaft to cause it to operatethe movable breaker point in proper timed relationship .to the engineand other parts of the magneto.

In the past, cams similar to the cam 62 have generally been looselymounted on the supporting shaft. That is, the internal diameters of thecams have been made of such sizes and tolerances with respect to theexternal sizes and tolerances of the shaft surface that the cams areeasily slipped onto the shafts during assembly. During the initialperiod of use following the assembly of such a cam on a shaft, theangular movement of the cam relative to the shaft is satisfactorilylimited by the engagement of the key on the cam with the key way in theshaft, and usually no additional means are provided to prevent the camfrom moving limited distances axially of the shaft. Initially, thislooseness of the cam on the shaft has no adverse effect on the operationof the magneto. However, during use the shaft is subjected to relativelysevere torsional vibrations which cause the cam to vibrate on the shaft.This relative movement between the cam and the shaft in turn causesfretting and other Wear which gradually increases the angular loosenessbetween the oam and the shaft, as by wearing down the walls of the keyon the cam and/or the walls of the key way in the shaft. Eventually,this increased angular looseness of the cam caused by wear of the partsoften becomes sufiicient to effect the timing of the breaker pointoperation and to thereby cause erratic operation of the engine.

In accordance with the invention, the cam 62 is so constructed andassembled with the other parts of the breaker mechanism as to eliminatelooseness between it and the shaft and thereby to eliminate wear of thecam or shaft as a result of shaft vibration. More particularly, the cam,as best shown in FIGS. 3, 4 and 5, is annular in shape and comprised ofan annular wall 80 defining a radially inner surface 82 and a radiallyouter peripheral cam surface 84. The inner cam surface 82 is generallycylindrical in shape so as to conform to the cylindrical shaft portion70, and along one angular portion of the cam the outer cam surface 84defines a lobe 86 along which the cam surface 84 is spaced a greaterdistance from the axis of the cam. The angular extent of the lobe 86 mayvary, but in the illustrated case is shown to comprise approximately a70 segment of the cam. Along the remainder of the circumference of thecam the cam wall 80 has a substantially uniform radial thickness so thatthe cam surface 84 is spaced a uniform distance from the axis of the camat all points along such remaining or dwell portion of the cam. The lobe86 of the cam is engageable with the follower 60 during rotation of thecam with the shaft to cause the movable breaker point 54 to be firstmoved away from and then returned to engagement with the fixed breakerpoint 48. The relationship of the parts is further such that while thedwell portion of the cam passes the cam follower 60 the follower is heldout of engagement with the cam surface 84 by the co-engagement of thebreaker points, as best shown in FIG. 5, until the lobe 86 returns toalignment with the follower.

To obtain a tight mounting of the cam to the shaft, the cam is designedso that before its assembly with the shaft the diameter ofthe openingprovided by the inner surface 82 is slightly less than the outsidediameter of the portion of the shaft to which it is to be assembled,thereby necessitating a force fit between the cam and the shaft. Inaddition, the cam is provided with a weakened zone to permit the camwall 80 to fracture along an axial line and to thereby allow the cam tospread or radially expand to accommodate the shaft. This weakened zonemay be provided in a variety of ways and preferably extends over only avery short angular extent of the cam so that the cam wall is ofrelatively high strength along substantially the full extent of itscircumference. The cam is also preferably made from sintered metal inaccordance with conventional powdered metallurgy techniques. By somaking the cam, it may be made in a single one-step forming operationwith the weakened zone and key 78 formed integral therewith so -as toeliminate the need for additional machining operations. In making asintered metal cam, however, it has been found desirable to avoid anysharp corners or discontinuities adjacent the weakened zone since thesintered metal in the vicinity of such corners or discontinuities tendsto be of an inferior or porous quality. Therefore, when the cam is madeof a sintered metal the weakened zone is preferably defined by aV-shaped groove 88, as shown in FIGS. 3, 4 and 5, having a roundedbottom of substantial radius so that the metal making up the cam in thevicinity of the weakened zone follows relatively smooth contours. Also,as will be evident from FIG. 5, the groove 88 is located in the dwellportion of the cam so that the cam follower is not engageable with thecam surface on either side of the groove.

In fitting the cam 62 to the shaft 16, it is first placed on the taperedshaft portion 74 and then pressed into the cylindrical shaft portion 70with the key 78 of the cam received in the key way 76 of the shaft. Asthis pressing operation takes place, the engagement of the shaft surfacewith the inner surface 82 of the cam creates an outward pressure on thecam which fractures the cam along the weakened zone, as shown in FIG. 5,and allows the cam to spread by a slight separation of the ends of thecam defined by the fracture line, After the cam reaches its fullyassembled position on the shaft, it remains in its spread condition andthe natural resiliency of the cam material causes the inner surface 82of the cam to tightly grip the outer surface of the shaft with asufficient force to resist angular and axial movement of the camrelative to the shaft as a result of the torsional shaft vibrations.

The degree of the press fit between the cam and the shaft and theminimum thickness of the cam in the area of the weakened zone may varydepending on the cam material and the relative sizes of the shaft andcam. For a cam proportioned generally as shown in the applicationdrawing and having an internal diameter of approximately five-eighths ofan inch and a length of approximately one-half of an inch, it has beenfound, in connection with sintered metal cams, that a minimum thicknessof .0020 to .0030 of an inch in the weakened zone and a press fit offrom .001 to .005 of an inch provides satisfactory results.

The invention claimed is:

1. A generally annular cam for a breaker point mechanism which mechanismincludes a shaft over which said cam is fitted, a fixed breaker point, amovable breaker point movable between open and closed positions relativeto said fixed breaker point, means for biasing said movable breakerpoint toward said closed position, and a cam follower cooperating withsaid cam for moving said movable breaker point relative to said fixedbreaker point inresponse to the rotation of said cam with said shaft,said cam comprising a body having an annular wall defining a radiallyinner surface and a radially outer peripheral cam surface including atleast one lobe, said lobe during a portion of each revolution of saidcam being engageable with said cam follower to hold open said movablebreaker point, said wall having a short angular and axially extendingportion along which its radial thickness is substantially less thanelsewhere so as to provide a weakened zone along which said wall willfracture to produce an axially extending fracture line as a result ofradially outwardly directed pressure applied over said radially innersurface, said radially inner surface defining an opening sufficientlysmaller in size than said shaft so that said cam during assembly withsaid shaft is fractured along said weakened zone by the pressure exertedby said shaft on said radially inner surface and so that said cam isthereafter held in a slightly spread condition by said shaft to cause itto tightly grip the latter.

2. A generally annular cam for a breaker point mechanism as defined inclaim 1 further characterized by said cam being made from a sinteredmetal.

3. A generally annular cam for a breaker point mechanism as defined inclaim 2 further characterized by said weakened zone being provided by aV-shaped groove in said wall having a rounded bottom of substantialradius so as to present no sharp discontinuity or corner.

4. In a breaker mechanism the combination comprising a shaft havingacylindrical portion and a tapered portion located adjacent saidcylindrical portion and converging therefrom, said tapered portionhaving a major diameter equal to the diameter of said cylindricalportion, and a cam having an annular wall defining an axially extendingcylindrical bore for receiving said shaft and also defining a radiallyouter peripheral cam surface including at least one lobe, said wallhaving a generally axially extending groove therein defining a weakenedarea of minimum radial thickness, said cam bore before assembly of saidcam with said shaft having a diameter larger than the minor diameter ofsaid tapered shaft portion and smaller than the diameter of saidcylindrical shaft portion, said cam being fractured along said weakenedarea and held in aslightlyspread condition as a result of being axiallyforced over said tapered shaftportion and onto said cylindrical shaftportion.

5. The combination defined in claim 4 further characterized by a fixedbreaker point, a movable breaker point movable between open and closedpositions relative to said fixed breaker point, means for biasing saidmovable breaker point toward said closed position, and a cam f-ollowercooperating with said cam for moving said movable breaker point relativeto said fixed breaker point in response to rotation of said cam withsaid shaft, said cam follower during a portion of each revolution ofsaid cam being engageable with said lobe to hold open said movablebreaker point and said cam wall being so shaped that during anotherportion of each revolution said cam follower is held out of engagementwith said cam by the engagement of said movable breaker pointwith saidfixed breaker point, said groove being located in that portion of saidcam wall traversed by said cam follower while held out of engagementwith said cam.

6. The combination as defined in claim 5 further characterized by saidcam being made from a sintered metal.

7. The combination as defined in claim 4 further characterized by saidshaft including an axially extending keyway, and said cam including anaxially extending key received in said keyway and integral with theremainder of said cam for angularly locating said cam on said shaft.

References Cited UNITED STATES PATENTS BERNARD A. GILHEANY, PrimaryExaminer.

I H. BROOME, G. MAIER, Assistant Examiners.

1. A GENERALLY ANNULAR CAM FOR A BREAKER POINT MECHANISM WHICH MECHANISMINCLUDES A SHAFT OVER WHICH SAID CAM IS FITTED, A FIXED BREAKER POINT, AMOVABLE BREAKER POINT MOVABLE BETWEEN OPEN AND CLOSED POSITIONS RELATIVETO SAID FIXED BREAKER POINT, MEANS FOR BIASING SAID MOVABLE BREAKERPOINT TOWARD SAID CLOSED POSITION, AND A CAM FOLLOWER COOPERATING WITHSAID CAM FOR MOVING SAID MOVABLE BREAKER POINT RELATIVE TO SAID FIXEDBREAKER POINT IN RESPONSE TO THE ROTATION OF SAID CAM WITH SAID SHAFT,SAID CAM COMPRISING A BODY HAVING AN ANNULAR WALL DEFINING A RADIALLYINNER SURFACE AND A RADIALLY OUTER PERIPHERAL CAM SURFACE INCLUDING ATLEAST LOBE DURING A PORTION OF EACH REVOLUTION OF SAID CAM BEINGENGAGEABLE WITH SAID CAM FOLLOWER TO HOLD OPEN SAID MOVABLE BREAKERPOINT, SAID WALL HAVING A SHORT ANGULAR AND AXIALLY EXTENDING PORTIONALONG WHICH ITS RADIAL THICKNESS IS SUBSTANTIALLY LESS THAN ELSEWHERE SOAS TO PROVIDE A WEAKENED ZONE ALONG WHICH SAID WALL WILL FRANCTURE TOPRODUCE AN AXIALLY EXTENDING FRACTURE LINE AS A RESULT OF RADIALLYOUTWARDLY DIRECTED PRESSURE APPLIED OVER SAID RADIALLY INNER SURFACE,SAID RADIALLY INNER SURFACE DEFINING AN OPENING SUFFICIENTLY SMALLER INSIZE THAN SHAFT SHAFT SO THAT SAID CAM DURING ASSEMBLY WITH SAID SHAFTIS FRACTURED ALONG SAID WEAKENED ZONE BY THE PRESSURE EXERTED BY SAIDSHAFT ON SAID RADIALLY INNER SURFACE AND SO THAT SAID CAM IS THEREAFTERHELD IN A SLIGHTLY SPREAD CONDITION BY SAID SHAFT TO CAUSE IT TO TIGHTLYGRIP THE LATTER.